The explanation, I believe, lies in the nature of the climate before the commencement of the Glacial period.

At this, the newer Pliocene period, the majority of the inhabitants of the world were specifically the same as now, and we have good reason to believe that the climate was warmer than at the present day.

Hence, we may suppose that the organisms which now live under latitude 60 degrees, lived during the Pliocene period further north, under the Polar Circle, in latitude 66-67 degrees; and that the present arctic productions then lived on the broken land still nearer to the pole.